French patent application number 88/12668 in the name of the same inventor and entitled "Dispositif de raccordement de ligne d'abonne" (Subscriber line connection circuit) filed Sept. 28, 1988, describes a circuit connected via a transformer to the subscriber line, and which does not include line current limitation.
European patent number 0 096 756 entitled "Joncteur d'abonne electronique" (Electronic subscriber interface circuit) describes a circuit which can only be made in the form of an integrated circuit because of its complexity and its use of current mirrors. In addition, it is necessary to add parallel protective devices on the line in order to ensure that the voltages on the line wires never exceed the power supply voltages of the integrated interface circuit under any circumstances, with such excess voltages being due to transients affecting the line. This interface circuit does not include metallic isolation between the 0V potential of the battery and logic ground, and as a result, even when the line is affected by transients, it is imperative that there should never be a potential difference of more than two volts between the 0V potential and logic ground.
The object of the invention is to provide an electronic subscriber connection circuit which avoids the drawbacks of the prior art electronic interface circuit.
Another object of the invention is to provide a connection circuit using discrete components and suitable for being implemented using surface-mount technology. Another object of the invention is to provide a low consumption connection circuit.
Another object of the invention is to provide a connection circuit which does not require parallel protective devices other than the voltage-dependent resistors (varistors) that already exist in the subscriber line.